A thin film transistor (TFT) has been widely used as, for example, (i) a circuit element for switching of a pixel in a liquid crystal display device (LCD) or (ii) a circuit element included in a driver of an LCD. In recent years, in order to achieve improvements such as a large screen, high definition, and high frame rate which are demanded of LCDs, TFTs are also increasingly demanded to achieve high performance and high reliability.
In accordance with such growing demands for high performance and high reliability of TFTs, types of TFTs are diversifying depending on types of semiconductors which can be used to form channel layers. Among such TFTs, (i) mass production techniques have been established for a single crystal silicon TFT, an amorphous silicon (a-Si) TFT, and a polycrystalline silicon (poly-Si) TFT and (ii) research and development is actively carried out with respect to a microcrystalline silicon (μc-Si) TFT, an oxide TFT, and an organic TFT.
Patent Literature 1 discloses a configuration of and a method for manufacturing a TFT in which a transparent conductive oxide semiconductor such as ZnO is used as a channel layer. The oxide semiconductor can be formed into a film at a low temperature and is transparent with respect to visible light. It is therefore possible to form a flexible and transparent TFT on a substrate such as a plastic plate and a film.
FIG. 14 is a cross sectional view illustrating a configuration of a conventional bottom-gate (inversely staggered) TFT. In the TFT, a gate electrode 102 is provided on a substrate 101, and thereon a first insulating film 103, an oxide semiconductor layer 104 serving as a channel layer, a second insulating film 105 serving as an etching stop layer, a source electrode 106, and a drain electrode 107 are provided.
In a case where the oxide semiconductor layer 104 is made of an amorphous oxide containing In, Zn, and O, the oxide semiconductor layer 104 can be prepared at a room temperature. Therefore, all film formation processes can be carried out at the room temperature, provided that the insulating films are formed by sputtering. The substrate can be a plastic substrate, a plastic film, or the like.
Patent Literature 1 also discloses that the source electrode 106 and the drain electrode 107 can be formed by patterning carried out by wet etching as well as dry etching, because the second insulating film 105 protects a channel region.
Note that Patent Literature 2 discloses a single transistor optical sensor circuit, which will be described later as an embodiment.
Patent Literature 3 discloses a circuit board in which (i) a photoelectric transducer having a light receiving section formed by an amorphous semiconductor and (ii) a switching element having a semiconductor layer formed by a polycrystalline semiconductor are provided on a light-transmitting substrate.
Patent Literature 4 discloses a liquid crystal display device including (i) a pixel switch formed by a non-single crystalline semiconductor and (ii) a peripheral driving circuit for driving the pixel switch, which peripheral driving circuit is formed by a single crystalline semiconductor. The pixel switch and the peripheral driving circuit are provided in a pixel display section.